1. Field of the Invention:
This invention pertains to electrical circuits for use with detection devices and, in particular, to circuits which provide electrical power to active, power-consuming detection devices and which return supervisory or trouble signals as well as alarm signals to the control panel of a detection and alarm system.
2. Description of the Prior Art:
Detection and alarm systems presently available in the prior art use one of two basic electrical circuits depending on the type of detection devices used in the system. Systems using passive or thermal detection devices utilize a two wire circuit loop for providing alarm signals to a control panel. Thermal detection devices are normally passive devices which include an electrical switch in a normally open position. In such devices provision is often made for a limited flow of current to provide for supervision of the continuity and the presence of devices in the loop. The presence of heat closes a switch, thereby increasing the flow of current and sending an alarm signal to the control panel.
In the present state of the art, systems utilizing active, power-consuming detection devices or combinations of active and passive devices require a four-wire circuit, two wires being used to supply electrical power to the detection devices and two wires being used for supervisory or trouble signals and alarm signals.
The circuit of the present invention provides a three wire circuit which accomodates active or passive detection devices, or combinations thereof, and furnishes both supervisory or trouble signals as well as alarm signals to the control panel. Thus it uses one less wire than normal, simplifying installation and reducing costs while maintaining an improved system performance because the control unit may easily distinguish the power being used to operate the detectors from the power used to operate the alarm system.